Recently, lithium secondary batteries are mainly used as a power source of mobile IT devices such as cellular phones and the need for lithium secondary batteries having high performance and high capacity is increasing in line with the increase of demand for electric vehicles (plug-in vehicles) and energy storage systems (ESS).
Lithium secondary batteries have a principle of producing electricity via the transfer of lithium ions (Li+) which are present in an ionic state from a positive electrode to a negative electrode during discharge and the transfer thereof from a negative electrode to a positive electrode during charge, and at this time, via the transfer of electrons with the lithium ions.
As the positive electrode active materials of lithium secondary batteries, various metal oxides such as a layered oxide (LiCoO2, LiNi1−x−yCoxMnyO2 (0<x<1, 0<y<1)), a spinel oxide (LiMn2O4) and an olivine oxide (LiFePO4) are used.
Recently, in order to further improve the electrochemical properties of such positive electrode active materials for a lithium secondary battery, research on the change of the composition of the positive electrode active materials, the control of a particle size, or the surface modification technique of a positive electrode active material is being diversely conducted.
The surface modification technique of the positive electrode active material can effectively improve defects concerning the deterioration of battery properties due to side-reactions by the direct contact of the positive electrode active material with an electrolyte and thermal stability, and is reported as an important technique for developing a high capacity/high energy material.
However, a surface modification material used for the modification of the surface of the positive electrode active material mainly includes metal oxides which are chemically stable but have low electric or ionic conductivity, and the movement of lithium ions is limited, thereby possibly generating defects of decreasing a capacity.
Accordingly, development on the research of a surface modification material and a surface modification method, by which high lithium ion conductivity may be attained, and a positive electrode active material may be imparted with structural stability, is urgent.